Stirling engine thermal system improvements

ABSTRACT

A thermal cycle engine having a heat exchanger for transferring thermal energy across the heater head from a heated external fluid to the working fluid. The heat exchanger has a set of heat transfer pins each having an axis directed away from the cylindrical wall of the expansion cylinder. The height and density of the heat transfer pins may vary with distance in the direction of the flow path, and the pin structure may be fabricated by stacking perforated rings in contact with a heater head. Ribs are provided interior to the heater head to enhance hoop strength and thermal transfer.

The present application is a divisional application of U.S. applicationSer. No. 09/884,436, now U.S. Pat. No. 6,694,731 filed Jun. 19, 2001,itself a continuation-in-part of U.S. application Ser. No. 09/517,245,now U.S. Pat. No. 6,381,958 filed Mar. 2, 2000, itself acontinuation-in-part application of U.S. application Ser. No.09/115,383, filed Jul. 14, 1998, and issued May 16, 2000 as U.S. Pat.No. 6,062,023, and a continuation-in-part also of Ser. No. 09/115,381,filed Jul. 14, 1998 and now abandoned, claiming priority from U.S.provisional application No. 60/052,535, filed Jul. 15, 1997, all ofwhich applications are herein incorporated by reference.

TECHNICAL FIELD

The present invention pertains to improvements to thermal components ofa Stirling cycle heat engine and more particularly to heat transfersurfaces such as the heater head.

BACKGROUND OF THE INVENTION

Stirling cycle machines, including engines and refrigerators, have along technological heritage, described in detail in Walker, StirlingEngines, Oxford University Press (1980), incorporated herein byreference. The principle underlying the Stirling cycle engine is themechanical realization of the Stirling thermodynamic cycle:isovolumetric heating of a gas within a cylinder, isothermal expansionof the gas (during which work is performed by driving a piston),isovolumetric cooling, and isothermal compression.

Additional background regarding aspects of Stirling cycle machines andimprovements thereto are discussed in Hargreaves, The Phillips StirlingEngine (Elsevier, Amsterdam, 1991) and in co-pending U.S. patentapplications Ser. No. 09/115,383, filed Jul. 14, 1998, and Ser. No.09/115,381, filed Jul. 14, 1998, which reference and both of whichapplications are herein incorporated by reference.

The principle of operation of a Stirling engine is readily describedwith reference to FIGS. 1 a–1 e, wherein identical numerals are used toidentify the same or similar parts. Many mechanical layouts of Stirlingcycle machines are known in the art, and the particular Stirling enginedesignated generally by numeral 10 is shown merely for illustrativepurposes. In FIGS. 1 a to 1 d, piston 12 and a displacer 14 move inphased reciprocating motion within cylinders 16 which, in someembodiments of the Stirling engine, may be a single cylinder. A workingfluid contained within cylinders 16 is constrained by seals fromescaping around piston 12 and displacer 14. The working fluid is chosenfor its thermodynamic properties, as discussed in the description below,and is typically helium at a pressure of several atmospheres. Theposition of displacer 14 governs whether the working fluid is in contactwith hot interface 18 or cold interface 20, corresponding, respectively,to the interfaces at which heat is supplied to and extracted from theworking fluid. The supply and extraction of heat is discussed in furtherdetail below. The volume of working fluid governed by the position ofthe piston 12 is referred to as compression space 22.

During the first phase of the engine cycle, the starting condition ofwhich is depicted in FIG. 1 a, piston 12 compresses the fluid incompression space 22. The compression occurs at a substantially constanttemperature because heat is extracted from the fluid to the ambientenvironment. The condition of engine 10 after compression is depicted inFIG. 1 b. During the second phase of the cycle, displacer 14 moves inthe direction of cold interface 20, with the working fluid displacedfrom the region of cold interface 20 to the region of hot interface 18.This phase may be referred to as the transfer phase. At the end of thetransfer phase, the fluid is at a higher pressure since the workingfluid has been heated at constant volume. The increased pressure isdepicted symbolically in FIG. 1 c by the reading of pressure gauge 24.

During the third phase (the expansion stroke) of the engine cycle, thevolume of compression space 22 increases as heat is drawn in fromoutside engine 10, thereby converting heat to work. In practice, heat isprovided to the fluid by means of a heater head 100 (shown in FIG. 2)that is discussed in greater detail in the description below. At the endof the expansion phase, compression space 22 is full of cold fluid, asdepicted in FIG. 1 d. During the fourth phase of the engine cycle, fluidis transferred from the region of hot interface 18 to the region of coldinterface 20 by motion of displacer 14 in the opposing sense. At the endof this second transfer phase, the fluid fills compression space 22 andcold interface 20, as depicted in FIG. 1 a, and is ready for arepetition of the compression phase. The Stirling cycle is depicted in aP-V (pressure-volume) diagram as shown in FIG. 1 e.

Additionally, on passing from the region of hot interface 18 to theregion of cold interface 20, the fluid may pass through a regenerator134 (shown in FIG. 2). Regenerator 134 is a matrix of material having alarge ratio of surface area to volume which serves to absorb heat fromthe fluid when it enters hot from the region of hot interface 18 and toheat the fluid when it passes from the region of cold interface 20.

Stirling cycle engines have not generally been used in practicalapplications due to such practical considerations as efficiency,lifetime, and cost which are addressed by the instant invention.

SUMMARY OF THE INVENTION

In accordance with preferred embodiments of the present invention, thereis provided a method for fabricating heat transfer protuberances, suchas for the heater head or cooler of a thermal cycle engine, wherein theheat transfer protuberances conduct heat between an external fluid and aworking gas through a cylindrical wall where the working gas is interiorto the wall. The method includes casting of the cylindrical wall and theheat transfer protuberances in a single operation. The casting step mayinclude investment casting, sand casting, or die casting. The method mayalso include steps of fabricating a plurality of negative molds, eachmold being of a group of substantially parallel holes corresponding tothe heat transfer protuberances in the fabricated part. The plurality ofnegative molds is assembled to form a negative form for casting thecylindrical wall and heat transfer protuberances.

In accordance with further embodiments of the invention, a method isprovided for fabricating heat transfer pins for conducting heat from anexternal thermal source through a cylindrical wall where the method hasthe steps of integrally fabricating at least one backing panel and heattransfer pins having axes normal to the backing panel, and then bondingthe at least one backing panel to a structure in thermal contact withthe cylindrical wall. The step of integrally fabricating the at leastone backing panel may include either casting or injection molding thebacking panel. The step of bonding may include mechanically attachingthe panel to the heater head, brazing the panel of the array of heattransfer pins to the heater head, or transient liquid-phase bonding ofthe panel of the array of heat transfer pins to the heater head. Inaccordance with yet further embodiments of the invention, a method isprovided for enhancing efficiency of thermal transfer through a heaterhead to a working gas in a thermal cycle engine, the heater head havingan interior surface. The method includes the step of applying a layer ofhigh-thermal-conductivity metal to the at least one of the interior andexterior surfaces of the heater head.

An alternate embodiment of the invention provides an improvement to aheater head for a thermal cycle heat engine that has a substantiallycylindrical wall section. The improvement has a plurality of ribsinterior to the wall section for providing enhanced hoop strength. Otherimprovements to a heater head, in accordance with the invention, includea plurality of passages within the wall that extend parallel to acentral longitudinal axis and a substantially helical channel within thecylindrical wall section. An additional improvement includes a pluralityof ribs interior to the dome for providing enhanced dome strength. Aplurality of flow diverters may also be provided, extending transverselyfrom a hot sleeve disposed internally to, and concentrically with, thecylindrical wall section.

In accordance with a further aspect of the present invention, a heatexchanger is provided for transferring thermal energy from a heatedexternal fluid across a cylindrical wall. The heat exchanger has a setof staggered heat transfer protuberances, each heat transferprotuberance having an axis directed substantially away from thecylindrical wall, and a plurality of dividers disposed substantiallyalong the length of the cylindrical wall, for forcing fluid flow throughthe staggered heat transfer protuberances.

In accordance with yet a further aspects of the present invention, aheat exchanger is provided for transferring thermal energy from a heatedexternal fluid across a cylindrical wall, where the heat exchanger has aset of heat transfer protuberances with axes directed substantially awayfrom the cylindrical wall, and a backer for guiding the heated externalfluid in a flow path characterized by a direction substantially alongthe length of the cylindrical wall past the set of heat transferprotuberances. A gap between the backer and the cylindrical wall maydecrease in the direction of the flow path of the external fluid. Inother embodiments of the invention, the heat transfer protuberances havea surface area transverse to the flow path that increases in thedirection of the flow path. In other embodiments of the invention, theheat transfer pins may have a population density that increases in thedirection of the flow path. In yet other embodiments of the invention,at least one of the height and density of the heat transfer pins mayvary with distance in the direction of the flow path.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood by reference to thefollowing description, taken with the accompanying drawings, in which:

FIGS. 1 a–1 e depict the principle of operation of a prior art Stirlingcycle machine;

FIG. 2 shows a side view in cross section of the heater head andcombustion chamber of a thermal engine in accordance with a preferredembodiment of the present invention;

FIG. 3 shows a further cross section of the heater head and combustionchamber of FIG. 2 along a direction in which both interior-facing andexterior-facing thermal pins are evident, and includes heat transferpins lining the interior and exterior surfaces of the top of heaterhead, in accordance with an alternate embodiment of the invention;

FIG. 4 a is a radial view of a group of parallel pins viewed towards thecentral axis of a cylindrical heater head in accordance with anembodiment of the present invention;

FIGS. 4 b and 4 c are cross sections of the heater head of FIG. 4 ataken parallel and transversely, respectively, to the central axis ofthe heater head cylinder;

FIG. 4 d is a perspective view of a heat transfer pin array separatelycast for assembly to a heater head in accordance with an embodiment ofthe present invention;

FIG. 5 a shows a perspective top view of a heater head for mounting castsegments of heat transfer pin arrays such as shown in FIG. 4;

FIG. 5 b shows a perspective top view of a heater head with mounted castsegments of heat transfer pin arrays, with the pin backer removed toshow the heat transfer pins;

FIG. 5 c is a cross sectional side view of the heater head assembly ofFIG. 3 showing the placement of ceramic insulation between the heaterhead temperature sensors and the exhaust gas, in accordance with anembodiment of the invention;

FIG. 6 a is a cross sectional side view of a heater head assembly withexternal heat transfer pin fins shown as well as a pin backer parallelto the wall of the heater head cylinder;

FIGS. 6 b–6 d plot the rate of heat transfer, heat transfer coefficient,and gas temperature, respectively, as a function of distance from thetop of the heat exchanger of FIG. 6 a;

FIG. 6 e is a cross sectional side view of a heater head assembly withexternal heat transfer pin fins shown as well as a pin backer parallelto the wall of the heater head cylinder;

FIGS. 6 f–6 h plot the rate of heat transfer, heat transfer coefficient,and gas temperature, respectively, as a function of distance from thetop of the heat exchanger of FIG. 6 e;

FIG. 6 i is a cross sectional side view of the heater head assembly ofFIG. 3 (with several heat transfer pins shown schematically for clarity)showing a typical gradient of temperatures as working fluid is driveninto the regenerator of a Stirling cycle engine in accordance with anembodiment of the present invention;

FIGS. 7 a–7 d depict the application of heat transfer pin rings toprovide for thermal transfer between fluids and a heater head inaccordance with an embodiment of the present invention;

FIG. 8 a plots strength curves (left-hand ordinate) and elongation(right-hand ordinate) as a function of temperature for a typical nickelalloy;

FIG. 8 b shows plots of creep rate vs. stress for a typical nickel alloyfor three temperatures between 1500° F. and 1700° F.;

FIG. 9 is a cross-sectional view of a heater head with internal ribbingin accordance with an embodiment of the present invention;

FIG. 10 is a partial cross-section of a heater head having internalribs, such as shown in FIG. 9, further showing an expansion cylinder hotsleeve with flow diverters in accordance with embodiments of the presentinvention;

FIGS. 11 a and 11 b are cross-sectional views of a thermal cycle engineheater head having uninterrupted tubes parallel to the outside walls inaccordance with an alternate embodiment of the present invention;

FIGS. 12 a and 12 b are cross-sectional views of a heater head for athermal engine having interrupted tubes parallel to the outside walls inaccordance with an alternate embodiment of the present invention;

FIGS. 13 a and 13 b are cross-sectional views of a heater head for athermal engine having helical fins along the interior of the outsidewalls in accordance with an alternate embodiment of the presentinvention;

FIG. 14 a is a side view of a core assembly for casting a heater headfor a thermal engine having helical fins along the interior of theoutside walls and a ribbed dome in accordance with an alternateembodiment of the present invention;

FIG. 14 b is a cross section, viewed downward transverse to the centralaxis, of the ribbed dome of a core assembly as shown in FIG. 14 a forcasting a heater head in accordance with an alternate embodiment of thepresent invention; and

FIG. 15 is a perspective view of the core assembly of FIG. 14 a assemblyfor casting a heater head for a thermal engine having helical fins alongthe interior of the outside walls and a ribbed dome in accordance withan alternate embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 2, a cross-sectional view is shown of the expansionvolume 98 of a thermal cycle engine, shown for illustrative purposes asa Stirling cycle engine designated generally by numeral 96, and of thecorresponding thermal control structures. Heater head 100 issubstantially a cylinder having one closed end 120 (otherwise referredto as the cylinder head) and an open end 118. Closed end 120 is disposedin a combustion chamber 122 defined by an inner combustor structure 110.Hot combustion gases in combustion chamber 122 are in direct thermalcontact with heater head 100 and thermal energy is transferred byconduction from the combustion gases to the heater head and from theheater head to the working fluid of the thermal engine, typicallyhelium. Other gases such as nitrogen, for example, or mixtures of gases,may be used within the scope of the present invention, with a preferableworking fluid having high thermal conductivity and low viscosity.Non-combustible gases are also preferred. Heat is transferred from thecombustion gases to the heater head as the combustion gases flow alongthe outside surface of closed end 120 within a gas flow channel 113.

Expansion volume 98 is surrounded on its sides by expansion cylinderliner 115, disposed, in turn, inside heater head 100 and typicallysupported by the heater head. The expansion piston 121 travels along theinterior of expansion cylinder liner 115. As the expansion pistontravels toward closed end 120 of heater head 100, the working fluidwithin the heater head is displaced and caused to flow through flowchannels defined by the outer surface of the expansion cylinder liner115 and the inner surface of heater head 100.

The overall efficiency of a thermal engine is dependent in part on theefficiency of heat transfer between the combustion gases and the workingfluid of the engine. One method known in the art for transferring heatefficiently from the combustion gases in combustion chamber 122 to theworking fluid in expansion volume 98 requires a plurality of heatingloops (not shown in FIG. 2, as they form no part of the specificembodiment shown there) that extend beyond the heater head and into thecombustion chamber.

In accordance with embodiments of the present invention, protuberances,such as fins or pins, may be used to increase the interfacial areabetween the hot fluid combustion products and the solid heater head soas to transfer heat, in turn, to the working fluid of the engine. Heaterhead 100 may have heat transfer pins 124, here shown on the interiorsurface of heater head 100, in the space between the heater head andexpansion cylinder liner 115. Additionally, as shown in FIG. 3 in across section of Stirling cycle engine 96 taken along a differentdiameter of expansion volume 98 from that of FIG. 2, heat transfer pins130 may also be disposed on the exterior surface of heater head 100 soas to provide a large surface area for the transfer of heat byconduction to heater head 100, and thence to the working fluid, fromcombustion gases flowing from combustor 122 past the heat transfer pins.Dashed line 131 represents the longitudinal axis of the expansioncylinder. FIG. 3 also shows heat transfer pins 133 lining the interiorand exterior surfaces of the top of heater head 100, in accordance withan alternate embodiment of the invention. Interior-facing heat transferpins 124 serve to provide a large surface area for the transfer of heatby conduction from heater head 100 to working fluid displaced fromexpansion volume 98 by the expansion piston and driven throughregenerator chamber 132. Depending on the size of heater head 100,hundreds or thousands of inner heat transfer pins 124 and outer heattransfer pins 130 may be desirable.

One method for manufacturing heater head 100 with heat transfer pins 124and 130 includes casting the heater head and pins (or otherprotuberances) as an integral unit. Casting methods for fabricating theheater head and pins as an integral unit include, for example,investment casting, sand casting, or die casting.

While the use of pin fins is known for improving heat transfer between asurface and a fluid, the integral casting of radial pin fins on thecylindrical heater head of a Stirling engine has not been practiced norsuggested in the art, despite the fact that casting the heater head andit's heat exchange surfaces in a single step is one of the most costeffective methods to produce a heater head. The difficulty encounteredin integral casting of radial pin fins is discussed further below. A pinfin that could be cast as part of cylindrical wall would allow theinexpensive fabrication of a highly effective heater head and/or coolerfor a Stirling engine.

Castings are made by creating negative forms of the desired part. Allforms of production casting (sand, investment and injection) involvesforming extended surfaces and details by injecting material into a moldand then removing the mold from the material leaving the desirednegative or positive form behind. Removing the mold from the materialrequires that all the extended surfaces are at least parallel. In fact,good design practice requires slight draft on these extended surfaces sothat they release cleanly. Forming radial pins on the outside or insideof a cylinder would require the molds to contain tens or hundreds ofparts that pull apart in different directions. Such a mold would be costprohibitive.

In accordance with the present invention, pins or fins may be cast ontothe inside and outside surface of Stirling heat exchangers usingproduction sand, investment or metal injection casting methods.Referring to FIGS. 4 a–4 d, and, first, to FIG. 4 a, pins 2002 arearranged into several groups 2008 of parallel pins 2002 aroundcylindrical wall 2010 of heater head 100, shown in cross sectionparallel to the central axis in FIG. 4 b and in cross section transverseto the central axis, in FIG. 4 c. It should be noted that the technologyherein described may advantageously be applied more generally in anyother heat exchanger application. All the pins 2002 in each group 2008are parallel to each other. Only the pins 2002 in the center of thegroup are truly radial. The pins on the outside of the group, such asthose designated by numeral 2004 in FIG. 4 c, are angled inward from alocal radius such as to be substantially parallel to a radial line 2012toward the center of the group. In addition, the pins on the outside ofthe group are preferably longer, typically by a small amount, than pinscloser to the center of the group. However, the heat transfer onlychanges only slightly from the center of the group to the outside in theembodiment depicted in FIGS. 4 a–4 c in which 5 groups 2008 of parallelpins provide approximately radial pin fins around cylinder 2010.

In the casting process in accordance with preferred embodiments of theinvention, positive or negative molds of each group of parallel fins areformed in a single piece. Several mold pieces are then assembled to formthe negative form for a sand casting. In investment mold casting, thewax positive can be formed in an injection mold with only a handful ofseparate parts that pull apart in different directions. The resultingmold is formed at an acceptable cost, thereby making production of a pinfin heater head economically practical.

Casting of a heater head having protuberances, such as pins, extendingto the interior and exterior of a part with cylindrical walls may beachieved, in accordance with embodiments of the present invention, byinvestment, or lost-wax, casting, as well as by sand casting, diecasting, or other casting processes. The interior or exteriorprotuberances, or both, may be integrally cast, in accordance with theteachings of this invention, as part of the head.

While typically more cheaply accomplished than machining or assembly ofthe pin arrays, casting pin arrays may still have attendant difficultiesand substantial costs. Additionally, the casting process may result in aheater head that is less than fully densely populated with pins, thusincreasing the fraction of gases failing to collide with the heater headsurface and reducing the efficiency of heat transfer.

An alternate method for populating the surfaces of heater head 100 withheat transfer pins, in accordance with other embodiments of theinvention, entails fabrication of heater 100 and arrays of heat transferpins in separate fabrication processes. An array 150 of heat transferpins 152 may be cast or injection molded with panel 154 resulting in anintegral backing panel structure shown in FIG. 4 d. Pin arrays 150,after casting or molding, are mounted to the inner and outer surfaces ofthe heater head by a high temperature braze. Thus, a more denselypopulated head with a resultant low rate of gas leakage past the pinsmay advantageously be achieved. In other embodiments, panels 154 may besecured by various mechanical means to the heater head.

Transient liquid-phase (TLP) bonding, as described, for example, in theAerospace Structural Metals Handbook, Code 4218, p. 6 (1999) isparticularly advantageous for brazing the panels to the head, sincenickel based superalloys, typically employed for fabrication of thehead, is difficult to weld by conventional processes, and operates in ahigh stress and high temperature environment. Advantages of TLP bondingin this application are that the parts braced by TLP are effectivelywelded using the parent material and have nearly the same tensilestrength properties as integrally cast parts. TLP bonds do not remelt atelevated temperatures, whereas typical brazes will remelt at the brazingtemperature. This is of particular significance in the case ofcontinuous operation at elevated temperatures where temperatureexcursions may occur, as in the present application.

The panels 154 of pins may be attached to the interior or exterior ofeither the heater head or the cooler by other means. In one alternativeembodiment, the panel may be mechanically attached into slots at itslateral edges. The slots are provided in dividers 506 (described in thefollowing discussion). In another embodiment, the panels are attached tothe heater head or cooler by brazing. In yet another embodiment, thepanels are attached to the heater head or cooler by sintering the panelsto the cylindrical walls of the heater head or cooler.

Dividers 506, as shown in FIGS. 4 c, 5 a, and 5 b, may advantageouslyimprove the heat transfer rate of the pin fin panels. Additionally, theymay provide a convenient location for locating temperature sensors.Lastly, the dividers may advantageously provide a convenient structureto which to attach panels of pins to the heater head, in one embodiment,and a parting line for casting operations, in accordance with a furtherembodiment.

Dividers 566 may serve to improve the thermal effectiveness of the pinfin arrays in the following manner. Referring, once again, to FIG. 4 a,the rate of heat transfer for a fluid flowing through staggered pin finsis significantly higher than for fluid flowing through aligned pin fins.Fluid approaching a staggered pin array 2008 would travel at a 45-degreeangle to an axial path along the length of the cylinder, with the skewdirection designated by numeral 2014. In order to provide for improvedthermal transfer, dividers 506 are provided, in accordance withpreferred embodiments of the invention, to force the fluid flow throughthe staggered array of pin fins along a path designated by numeral 2012.In addition to forcing the flow to travel axially, the dividers provideconvenient interfaces and joining planes for the casting molds describedabove.

In certain embodiments of the invention, individual arrays 150, eachwith its associated panel segment 154, comprise arcuate fractions of thecircumferential distance around the heater head. This is apparent in thetop view of the heater head assembly shown in perspective in FIG. 5 a.Cylinder head 120 is shown, as is exterior surface 502 of the heaterhead. Backer segments supporting arrays of heat transfer pins are notshown but are inserted, during assembly, in spaces 504 surroundingexterior surface 502 of the heater head. Between successive heattransfer pin array segments are trapezoidal dividers 506 which arebaffled to block the flow of exhaust gases in a downward directionthrough any path other than past the heat transfer pins.

In one embodiment, flow dividers 506 include structures for mechanicallyretaining the panel segments 154 during assembly, before brazing, orsimply to mechanically retain the panels 154 against heater head 502.

In order to maximize engine power, the hottest part of the heater headis preferably at the highest temperature allowed, considering themetallurgical creep and tensile strength, stress, and appropriatefactors of safety. Maintaining the hottest part of the heater head atthe highest temperature requires measuring the temperature of thehottest part of the heater head. The dividers provide a convenientlocation and routing for temperature sensors on the heater had to anyaxial location along the pin fin arrays. Hot gas flow path 113 (shownalso in FIG. 2), is defined, on the outside, by gas flow channel cover140. Since exhaust gases do not flow through dividers 506, a temperaturesensor, such as thermocouple 138 (shown in FIGS. 2 and 5 c) isadvantageously disposed in divider 506 in order to monitor thetemperature of heater head 100 with which the temperature sensor is inthermal contact. The position of pin arrays 150 and temperature sensor138 mounted within divider 506 is shown more clearly in the view of FIG.5 b in which the pin backer has been removed.

Temperature sensing device 138 is preferably disposed within divider 506as depicted in FIG. 5 b. More particularly, temperature sensing tip 139of temperature sensor 138 is preferably located in the slotcorresponding to divider 506 as nearly as possible to cylinder head 120in that this area is typically the hottest part of the heater head.Alternatively, temperature sensor 138 might be mounted directly tocylinder head 120, however location of the sensor in the slot, asdescribed, is preferred. Engine performance, in terms of both power andefficiency, is highest at the highest possible temperature, yet themaximum temperature is typically limited by metallurgical properties.Therefore, sensor 138 should be placed to measure the temperature of thehottest, and therefore the limiting, part of the heater head.Additionally, temperature sensor 138 should be insulated from combustiongases and walls of divider 506. One embodiment provides ceramicinsulation 142 between the temperature sensor and the combustion gasesand divider walls, as shown in FIG. 5 c. The ceramic can also form anadhesive bond with the walls of the divider to retain the temperaturesensor in place. Electrical leads 144 of temperature sensor 138 shouldalso be electrically insulated.

The power of the engine is limited, among various factors, by thethermal efficiency of the heater head. This thermal efficiency depends,in turn, on the fin efficiency of the pin fins. Requirements of highcreep strength and oxidation resistance at very high operatingtemperature make the use of high nickel alloys preferable. Theefficiency of the interior fins may be advantageously increased byapplying a layer of highly thermally conductive metal, such as nickel orcopper, of thickness greater than 0.001 in. and preferably about 0.005in., to interior surface 148 of heater head 120, by deposition orplating, or other application method. Alternatively, a similar coatingmay be applied to the exterior surface, in accordance with anotherembodiment of the invention.

In order to keep the size of a Stirling cycle engine small, the heatflux from the combustion gas through the heater head is preferablymaximized. Referring now to FIG. 6 a, heat transfer into the Stirlingheater head and therefore the Stirling engine power is limited by themaximum temperature of the heater head, as discussed above. In order tomaximize power, the heater head is preferably fabricated from the familyof high nickel alloys, commonly known as super alloys, such as Iconel600 (having a maximum temperature T_(max)=800° C. before softening,Iconel 625 (T_(Max)=900° C.), Iconel 754 (T_(max)=10800° C.), orHastelloy GMR 235 (T_(max)=935° C.). A heater head dome 2216 with theheat exchange surface located on the cylindrical wall 2010 is limited bythe temperature at the top 2200 of heat exchanger area 2218. Here thegas temperatures are the highest (typically greater than 1500° C.), asgoverned by the temperature of flame 2220 and the rate of heat transferfrom the hot combustion gases on the outside and cooling from theworking gas on the inside. The rate of heat transfer, shown as afunction of distance from the top 2200 of the heat exchanger in FIG. 6b, is a function of the burner power and air flow. The burner power andtherefore the engine power may be increased if the heat transfer betweenthe combustion gases and the heater head is reduced at the place wherethe gases are the hottest.

At the same time as the top 2200 of the heat exchange surface is gettingtoo hot, the amount of heat transfer at the bottom 2210 of the heatexchanger 2218 is too low. The gas cools rapidly as it gives up thermalenergy to the heater head, as plotted in FIG. 6 d as a function ofdistance from the top of the heat exchanger, so that by the time the gasexits the heat exchanger the gas temperature 2202 is near the headtemperature 2214 and the rate of heat transfer 2206 is nearing zero.

The most efficient heat exchanger would have high heat transfer over itsentire surface. This would maximize the amount of heat transferred tothe head for given maximum head temperature. The problem is that heattransfer coefficient 2204, plotted in FIG. 6 c as a function of distancefrom the top of the heat exchanger, is fairly constant, while the gastemperature 2204 is dropping rapidly.

A solution, in accordance with preferred embodiments of the presentinvention, is to vary the heat transfer coefficient to compensate forthe change in combustion gas temperature as the gas cools. Therefore,the heat transfer coefficient 2304, plotted in FIG. 6 f, preferablyincreases from a low at the top of the heater head heat exchanger to amaximum at the bottom. This can be accomplished in several waysencompassed by embodiments of the invention as now described.

Referring now to FIG. 6 h, pin fins 2320 or other heat transfer surfacesin the first embodiment are constant from the top 2300 to the bottom2310 of the heat exchanger 2318. A “fin backer” 2316 is formed on theoutside of the heater head heat exchanger that allows part of thecombustion gas 2308 to bypass the top 2300 of the heat exchanger. Thisuncooled combustion gas then gradually enters the heat exchanger 2318along its length. Allowing some of the combustion gas to bypass the topend 2300 of the heat exchanger and gradually adding it to the flow ofcombustion gas past subsequent pin fins evens out the heat transfer 2306to the head, as plotted in FIG. 6 e, for at least two reasons. First,more gas is forced through the lower part of the heat exchanger therebyincrease the flow rate toward the bottom 2310 and thus also increasingthe coefficient of heat transfer 2304 as plotted in FIG. 6 f. Second,fresh hot gas is continuously added to the heat exchanger and thusmaintains the combustion gas temperature 2302, as plotted in FIG. 6 g,at a higher average temperature relative to the head temperature 2314.

In other embodiments, the pin backer may have a constant inside diameterand the pin fin geometry may be varied along the length of the heatexchanger. The heat transfer coefficient will vary from a low at the topof the heat exchanger to maximum at the bottom. This can be accomplishedin a number of ways. In one embodiment the density of constant size pinfins increases from a low that the top of the heat exchanger to maximumat the bottom. In another embodiment, the pin height increases from thetop to the bottom of the heat exchanger. In another embodiment, spacingbetween the pins decreases from top to bottom. The ideal dimensions ofthe pin height, diameter and spacing depend on the particularapplication. The invention teaches that the heat transfer coefficient isto be increased from top to bottom of the heater head heat exchanger tocompensate for the decreasing combustion gas temperature, with theobjective being a more constant heat transfer rate along the heater head

The mechanical realization of the variable cross-section gas flowchannel 113, as described above, is also shown in FIG. 6 i. Thecross-sectional view of FIG. 6 i shows how tapered pin backer 146 allowssome of the hottest exhaust gas to bypass the pins near the top of theheater head. Pin backer 146 creates a narrowing annular gap on theoutside of the pins that progressively forces more and more of theexhaust gases into the pin heat exchanger. The temperature gradient fromthe top of the heater to the bottom of the hot section (beforeregenerator volume 132, shown in FIG. 2) has been reduced from as muchas 350° C. to 100° C. using a variable cross-section gas flow channel.

Another method for increasing the surface area of the interface betweena solid such as heater head 100 and a fluid such as combustion gases asdiscussed above is now described with reference to FIGS. 7 a–7 d. Aneffect analogous to that of fabricating heat transfer pins by casting orotherwise may be obtained by punching holes 160 into a thin annular ring162 shown in top view in FIG. 7 a and in side view in FIG. 7 b. Thethickness of ring 162, which may be referred to as a ‘heat transfer pinring’ is comparable to the thickness of the heat transfer pins discussedabove, and is governed by the strength of the heat-conductive materialat the high temperature of the combustion gases traversing holes 160.The shape and disposition of holes 160 within each ring is a matter ofdesign for a particular application, indeed, it is within the scope ofthe present invention and of any appended claims that holes 160 not besurrounded by solid material. The material of rings 162 is preferably anoxidation-resistant metal such as Inconel 625 or Hastelloy GMR 235,though other heat-conducting materials may be used. Rings 162 may beproduced inexpensively by a metal stamping process. Rings 162 are thenmounted and brazed, or otherwise bonded, to the outer surface heaterhead 100, as shown with respect to outer pin rings 164 in FIG. 7 c, andwith respect to inner pin rings 166 in FIG. 7 d. Additional rings may beinterspersed between the pin rings to control the vertical spacingbetween the pins. Expansion cylinder liner 115 is shown in the interiorof inner pin rings 166.

Heat transfer rings 162 may be advantageously applied to the interior ofthe heater head as well as to both the exterior and interior of thecooler of a thermal cycle engine. In these applications, the rings neednot be oxidation resistant. Materials including copper and nickel arepreferably used on the interior of the heater head, while the rings forthe cooler are preferably made of one of various high thermalconductivity materials including aluminum, copper, zinc, etc.

The total cross sectional area of the heat transfer pins taken in aslice perpendicular to cylinder axis 168 need not be constant, indeed,it is advantageously varied, as discussed in detail above, in referenceto FIG. 6.

The walls of the heater head must be sufficiently strong, at operatingtemperatures, to withstand the elevated pressure of the working gas. Itis typically desirable to operate Stirling cycle engines at as high aworking gas pressure as possible, thus, enabling the head to withstandhigher pressures is highly advantageous. In designing the heater head,it must be borne in mind that increasing the pressure at a givenoperating temperature typically requires increasing the heater head wallthickness in direct proportion. On the other had, thickening the heaterhead wall results in a longer thermal conduction path between theexterior heat source and the working gas.

Moreover, thermal conduction increases with heat exchanger surface area,thus thermal efficiency is increased by increasing the diameter of theheater head. Stress in the wall, however, is substantially proportionalto the diameter of the head, thus increasing the head diameter, at agiven temperature and interior gas pressure, requires increasing thewall thickness in direct proportion.

The strength considerations are tantamount at typical Stirling enginehead temperatures, in fact, they drive the maximum operatingtemperature, since, as discussed, efficiency increases with temperature.Both creep and ultimate tensile strengths of materials tend to fall offprecipitously when specified elevated temperatures are reached.Referring to FIG. 8 a, the yield strength at 0.2% offset and ultimatetensile strength are shown for the GMR 235 nickel alloy in typicalrepresentation of the qualitative behavior of nickel alloys. Similarly,in FIG. 8 b, it can be seen that the 0.01% per hour creep rate strengthof GMR 235 falls from 40 ksi to half as the temperature rises from 1500°F. to 1700° F.

Preferred embodiments of the present invention provide interior ribs (orhoops) 200 that enhance structural support of heater head 100, as shownin cross-section in FIG. 9. Ribs 200 are characterized by an interiorbore 202. The creep strength and rupture strength of heater head 100 isthus determined predominantly by an effective thickness 204 of theheater head and the interior bore diameter 202. Heat conduction throughthe heater head is not limited by thickness 204 since interveningsegments 206 of the head are narrower and provide enhanced heatconduction. Ribs 200 not only relieve hoop stresses on outer wall 208 ofhead 100 but additionally provide supplemental surface area interior tothe heater head and thus advantageously enhance heat transfer to theworking fluid.

Further advantages of providing ribs 200 interior to the heater headinclude reducing the temperature gradient across the head wall 208 for agiven rate of heat transfer, as well as allowing operation at higher hotend working temperatures. Additionally, by reducing the stressrequirements on the outer wall, alternative materials to nickel basedsuperalloys may be used, advantageously providing superior conductivityat reduced cost.

A cross section of heater head 100 with ribs 200 is further shown inFIG. 10. Dashed line 210 designates the central longitudinal axis of theexpansion cylinder. In accordance with embodiments of the invention,expansion cylinder hot sleeve 212 may have transverse flow diverters 214for directing the flow of working gas, represented by around 216, aroundcircumferential ribs 200 for enhancing heat transfer to the working gas.The additional width h of ribs 200 contributes to the hoop strength ofheater head 100, whereas heat transfer is governed predominantly by thenarrower thickness t of outer heater head wall 208. In typical Stirlingengine applications, while the heater head exterior may be run as hot as1800° F., ribs 200 that provide structure strength typically run nohotter than 1300° F.

Advantages of enhanced hoop strength concurrent with enhanced thermalconductivity, as discussed above with reference to FIG. 9 mayadditionally be obtained in accordance with several alternateembodiments. Referring to FIGS. 11 a and 11 b, cross sections are shownof a heater head 230, wherein tubular openings 232 run parallel toheater head wall 208. As shown in the cross sectional view of FIG. 11 b,taken along line AA, tubes 232 allow working gas to pass down the wall,enhancing heat transfer from outside the head to the working gas.Additionally, the wall 208 may be thicker, for the same rate of heattransfer, thus providing additional strength. Moreover, the thick wallsection 210 interior to passages 232 remains cooler than would otherwisebe the case, providing further additional strength. Heater head 230 ispreferably cast with tubular passages 232 which may be round in crosssection or of other shapes.

FIG. 12 a shows a further heater head 240, in accordance with otherembodiments of the invention, wherein tubular openings 232 run parallelto heater head wall 208 and are interrupted by openings that run out tothinner sections 242 of the heater head wall. As shown in the crosssectional view of FIG. 12 b, taken along line AA, tubes 232 allowworking gas to pass down the wall, enhancing heat transfer from outsidethe head to the working gas to a degree substantially enhanced over thatof the straight tube design shown in FIGS. 11 a and 11 b. Additionally,openings 244 provide additional area for removal of ceramic cores usedin the casting process to create such long, thin holes. Increased accessto the holes allows faster chemical leaching of the core in the courseof the manufacturing process.

FIG. 13 b shows yet another heater head 250, wherein ribs 252 aredisposed in a helix within heater head wall 208, thereby providing thewall with enhanced rigidity in both the circumferential and axialdirections. The working gas flows through the spiral 254 on a pathbetween the expansion piston and the heater head, on its way to theregenerator. FIG. 13 b shows a transverse cross section of the heaterhead of FIG. 13 a taken along line AA. It is also within the scope ofthe invention to employ a linear, or other, approximation to spiral 254,to obtain comparable advantages of stiffening and heat transfer.

Heater head 250 of FIGS. 13 a and 13 b is preferably fabricated bycasting. A side view of core assembly 260 for use in the casting processis shown in FIG. 14 a. It is additionally advantageous to provide ribsfor internal support of the dome of the heater head and to provideadditional heat exchange on the dome, thereby cooling the inner surfaceof the dome. The complementary core structure of the dome is shown inFIG. 14 b, and, in cross section, as viewed from the top, in FIG. 14 b.A perspective view of core assembly 260 is shown in FIG. 15.

The devices and methods described herein may be applied in otherapplications besides the Stirling engine in terms of which the inventionhas been described. The described embodiments of the invention areintended to be merely exemplary and numerous variations andmodifications will be apparent to those skilled in the art. All suchvariations and modifications are intended to be within the scope of thepresent invention as defined in the appended claims.

1. In a thermal cycle engine of the type having a piston undergoingreciprocating linear motion within an expansion cylinder, the expansioncylinder having a cylindrical wall and containing a working fluid heatedby conduction through a heater head of heat from an external thermalsource, the conduction characterized by a heat transfer coefficient, theimprovement comprising: a. a heat exchanger for transferring thermalenergy across the heater head from a heated external fluid to theworking fluid, the heat exchanger comprising a set of heat transferprotuberances, each heat transfer protuberance having an axis directedsubstantially away from the cylindrical wall of the expansion cylinder;and b. means for increasing the heat transfer coefficient in a directionof flow of the heated external fluid.
 2. A heat exchanger fortransferring thermal energy from a heated external fluid across acylindrical wall, the cylindrical wall characterized by a lengthdirection parallel to a central axis of the cylindrical wall, the heatexchanger comprising: a. a set of staggered heat transfer protuberances,each heat transfer protuberance having an axis directed substantiallyaway from the cylindrical wall; b. a plurality of dividers disposedsubstantially along the length of the cylindrical wall, for forcingfluid flow through the staggered heat transfer protuberances.
 3. A heatexchanger for transferring thermal energy from a heated external fluidacross a cylindrical wall characterized by an axial direction, the heatexchanger comprising: a. a set of heat transfer protuberances, each heattransfer protuberance having an axis directed substantially away fromthe cylindrical wall; and b. a backer for guiding the heated externalfluid in a flow path substantially along the axial direction of thecylindrical wall past the set of heat transfer protuberances, whereinthe spacing between the cylindrical wall and the backer varies.
 4. Aheat exchanger for transferring thermal energy from a heated externalfluid across a cylindrical wall characterized by an axial direction, theheat exchanger comprising: a. a set of heat transfer protuberances, eachheat transfer protuberance having an axis directed substantially awayfrom the cylindrical wall; and b. a backer for guiding the heatedexternal fluid in a flow path substantially along the axial direction ofthe cylindrical wall past the set of heat transfer protuberances,wherein a gap between the backer and the cylindrical wall decreases inthe axial direction.
 5. A heat exchanger for transferring thermal energyfrom a heated external fluid across a cylindrical wall characterized byan axial direction, the heat exchanger comprising: a. a set of heattransfer protuberances, each heat transfer protuberance having an axisdirected substantially away from the cylindrical wall; and b. a backerfor guiding the heated external fluid in a flow path substantially alongthe axial direction of the cylindrical wall past the set of heattransfer protuberances, wherein the heat transfer protuberances have asurface area transverse to the flow path that increases in the axialdirection.
 6. A heat exchanger for transferring thermal energy from aheated external fluid across a cylindrical wall characterized by anaxial direction, the heat exchanger comprising: a. a set of heattransfer protuberances, each heat transfer protuberance having an axisdirected substantially away from the cylindrical wall; and b. a backerfor guiding the heated external fluid in a flow path substantially alongthe axial direction of the cylindrical wall past the set of heattransfer protuberances, wherein the heat transfer pins have a populationdensity that increases in the axial direction.
 7. A heat exchanger fortransferring thermal energy from a heated external fluid across acylindrical wall characterized by an axial direction, the heat exchangercomprising: a. a set of heat transfer protuberances, each heat transferprotuberance having an axis directed substantially away from thecylindrical wall; and b. a backer for guiding the heated external fluidin a flow path substantially along the axial direction of thecylindrical wall past the set of heat transfer protuberances, wherein atleast one of the height and density of the heat transfer pins varieswith distance in the axial direction.